Fluorine-containing polymers are widely employed in the fields of automobile industries, semiconductor industries and chemical industries, since those polymers exhibit excellent chemical resistance, solvent resistance and heat resistance. Particularly fluorine-containing polymers comprising vinylidene fluoride (VdF) and/or tetrafluoroethylene (TFE) unit and a copolymerizable monomer unit such as hexafluoropropylene (HFP) or perfluoro(alkyl vinyl ether) (PAVE) exhibit characteristics of elastomer depending on a composition thereof, and therefore are used as an O-ring, gasket, hose, stem seal, shaft seal, diaphragm, electric wire, shock-absorbing material and the like which are used under severe environment. However fluorine-containing elastomers are generally expensive as compared with other kinds of rubbers, and therefore shortening of a cycle time at mold-processing and improvement in yield are strongly demanded. Accordingly, for the purpose of taking the pace of a method of compression molding requiring a long cycle time and a method of transfer molding giving a poor yield, there have been developed a method of injection molding enabling high cycle production and a method of integrated molding with other material which makes it possible to reduce length of a molding step. However many of polymers suitable for these molding methods of low cost are those having a relatively low viscosity, and thus conventional fluorine-containing elastomers cannot sufficiently meet the requirements of such methods.
On the other hand, such fluorine-containing elastomers are prepared by various polymerization methods such as a suspension polymerization method and an emulsion polymerization method. Of these fluorine-containing elastomers, those obtained by an emulsion polymerization method are in such a state that primary particles having an average particle size of several tens to hundreds of nanometers are emulsified and dispersed in water. Since it is difficult to directly filtrate the fine primary particles in this emulsified dispersion, usually there is employed a method of agglomerating the primary particles by adding a coagulant such as a metallic salt or an acid to obtain agglomerated (secondary) particles having an average particle size of several μm to thousands of μm and then filtrating the obtained particles to prepare fluorine-containing elastomers. Though this method can be used suitably for preparing fluorine-containing elastomers having a medium viscosity and a high viscosity, but in the case of preparing fluorine-containing elastomers having a low viscosity, there is a problem that the coagulated polymer sticks to a stirrer blade and a filter of equipment used in a separation step, resulting in blocking of equipment, and it is difficult to efficiently separate the polymer. In order to solve this problem, a method of treating in a state of low viscosity by freeze-coagulation is known, but there is a problem that a running cost is very high. Also it is usually difficult to completely remove water only by the above-mentioned filtration, and drying by heating is necessary after solid-liquid separation. For example, a green rubber is obtained by a method such as hot air drying with a chamber oven or melt-extrusion drying with an extruder. Even in such a process, in the case of a low viscosity rubber, there is a problem, in many cases, that contamination of equipment and equipment failure occur due to foaming and adhesion of a product. As mentioned above, because of not only a problem that dehydration and drying steps themselves are usually complicated but also the above-mentioned problems occurring in the case of usual treating method of a low viscosity rubber, it has been difficult to bring products to market.
Also it is known that reduction of metals contained in a fluorine-containing polymer is particularly preferable in the field of semiconductor producing equipment where cleanness is required, and for that purpose, it is effective not to use metals in all the preparation processes such as polymerization and coagulation. For example, for the purpose of avoiding mixing of a metal derived from a coagulant such as aluminum sulfate which is usually used in a coagulation step, a method of coagulation with an acid or by a freezing process is disclosed (for example, refer to WO 99/50139). However in the case of using an acid, since an acid is a dangerous chemical, it is not preferable from the viewpoint of working environment. Also equipment having corrosion resistance costs high, and further it is necessary to use devices subjected to special treatment such as lining with a resin in order to prevent elution of a metal from the equipment. In addition, in the case of a freezing process, there is a problem with a high running cost resulting from refrigerating. Also in the both cases, there is a problem that a special attention must be given to prevent mixing of a metal during transferring to a drying step.
For separating a fluorine-containing elastomer from an emulsion of fluorine-containing elastomer, a method of separating a polymer from a polymer being heavier than water and containing water in an amount of up to about 90% by using a specific separator is disclosed (for example, cf. JP53-147787A).
In addition, a method of coagulating and agglomerating fluoroelastomer particles in a slurry flow to be treated of fluoroelastomer and then supplying the obtained treated flow containing agglomerated fluoroelastomer to a mechanical dehydrating equipment is disclosed (for example, cf. JP58-176219A). However in this method, since a coagulant is used like a conventional method, complicated steps are required and an obtained fluoro rubber contains a metal. Therefore it is not preferable to use this method in the field of semiconductor where cleanness is required. Further it is known by persons skilled in the art that in treating a rubber having a low viscosity, to prevent blockage of equipment due to adhesion of a product, various treating conditions and means are required for the equipment.
Further there is disclosed a process for preparing a fluoro rubber by radical polymerization of vinylidene fluoride and an optional fluorine-containing monomer and/or non-fluorine-containing monomer under specific conditions in the absence of water, which is characterized in that the fluoro rubber is fed to an outlet at the base of a reactor and pushed out at a temperature at least corresponding to a reaction temperature (for example, cf. JP2001-508474A). However in JP2001-508474A, polymerization is carried out in the presence of a non-aqueous solvent, and there is no description with respect to an aqueous dispersion of a polymer.
Accordingly, at present there is no method of being capable of efficiently separating a fluorine-containing polymer and an aqueous solvent from an aqueous dispersion of a fluorine-containing polymer and obtaining a fluorine-containing polymer containing no metal.